The method and apparatus of the present invention are improvements over the autocirculation method and apparatus disclosed in prior U.S. Pat. Nos. 4,238,462 and 5,160,714 and is useful for gas-liquid mass transfer where a liquid is contacted with two different gases in separate contact zones. The series of reactions involved in catalytically oxidizing sulfur contaminants, such as hydrogen sulfide, to elemental sulfur using an iron chelate catalyst can be represented by the following reactions, where L generically represents the two or more particular ligands chosen to formulate the metal chelate catalyst admixture:H2S(gas)+H2O(liq.)→H2S(aqueous)+H2O(liq.)  (1)H2S(aqueous)→H++HS−  (2)HS−+2(Fe3+L2)→S(solid)+2(Fe2+L2)+H+  (3)
By combining equations (1) through (3) the resulting equation is:H2S(gas)+2(Fe3+L2)→2H++2(Fe2+L2)+S(solid)  (4)
In order to have an economical workable process for removing hydrogen sulfide from a gaseous stream when a polyvalent metal chelate admixture is used to effect catalytic oxidation of the hydrogen sulfide, it is essential that the ferrous iron chelate formed, as exemplified above, be continuously regenerated by oxidizing to ferric iron chelate on contacting the reaction solution with dissolved oxygen, preferably in the form of ambient air, in the same or in a separate contact zone. The series of reactions which take place in the oxidizer of my invention when regenerating the metal chelate catalyst can be represented by the following equations:O2(gas)+2H2O→O2(aqueous)+2H2O  (5)O2(aqueous)+2H2O+4(Fe2+L2)→4(OH−)+4(Fe3+L2)  (6)
By combining equations (5) through (6), the resulting equation (7) is:½O2+H2O+2(Fe2+L2)→2(OH−)+2(Fe3+L2)  (7)
And, when equations (4) and (7) are combined, the overall process can be represented by the following equation:H2S(gas)+½O2(gas)→S(solid)+H2O(liq.)  (8)
It will be evident from the foregoing equations that theoretically two moles of ferric iron must be supplied to the reaction zone in which the hydrogen sulfide gas is oxidized to form elemental sulfur for each mole of hydrogen sulfide gas treated, and in actual practice considerably more than the theoretical amount of iron is used. In a continuous process of removing hydrogen sulfide by contact with a catalytic ferric iron solution, the catalytic solution is circulated continuously between an absorber zone, where the H2S is absorbed by the catalytic ferric iron chelate solution, and the solution is reduced to ferrous iron and an oxidizer zone is used to oxidize the reduced ferrous iron back to the ferric iron state. In order to avoid using high concentrations of iron in the catalytic solution, the rate of circulation should be high.
The method and apparatus described in earlier autocirculation references has been commercially successful, but the commercial use of that method and apparatus suffers from several disadvantages including some lack of control of residence time for gas-liquid contact in each of the reaction zones and poor liquid flow control. U.S. Pat. No. 5,160,714 provides a method of contacting a liquid with different gases sequentially in separated mass transfer zones within a single vessel whereby the rate of liquid flow from one mass transfer zone to another is motivated by the difference in the aerated liquid density in a mass transfer zone and the non-aerated liquid density in a preceding liquid downcomer. This density difference acts as a “pump” to create a driving force. In this patent, it was contemplated that liquid flow rate would be controlled by adjusting the gas rate to one or more mass transfer zones only; however, this did not prove to be practical since the gas rates to the various mass transfer zones are generally governed by process requirements other than liquid flow rate. The amount and composition of sour gas entering the apparatus is always controlled by upstream processes and is thus independent of the operation of the apparatus. The operation of the apparatus must be able to adjust to the inlet sour gas condition. The amount of air injected into the oxidization zone is dependent on the amount of H2S contained in the sour gas, and the liquid circulation rate must, at a minimum, supply sufficient moles of iron to satisfy equation 3 in the reaction zone. If left uncontrolled, the actual solution circulation rate will be determined by the physical characteristics of the apparatus, and the aerated densities of the reaction zone and the oxidizer zone. If the solution circulation rate is too high, oxygen can be transferred from the oxidization zone into the reaction zone resulting in the production of unwanted byproducts, such as sulfates. If the solution circulation rate is too slow, insufficient iron will be supplied to the reaction zone to satisfy reaction 3, resulting in the formation and precipitation of iron sulfide.
To compensate for this lack of control, various items such as butterfly valves, flow restricting wedges and slide gates were installed in the liquid conduit line that recycled the spent reagent from the absorber to the oxidizer. Unfortunately, all of the proposed solutions to control liquid flow through the mass transfer zones proved to be impractical due to plugging caused by solids (i.e., elemental sulfur) entrained in the liquid.
My invention addresses the needs of those in the art and in particular provides an apparatus and process steps that alleviate all of the problems and difficulties of previous flow control devices. These and other advantages will become evident from the following more detailed description of the invention.